The removal of metal contaminants from aqueous wastes is an important environmental issue, and although the problem of removing toxic and heavy metals from waters has been addressed for many years, few effective treatment options are available that are not reagent and energy requirement intensive, and where the metal values are recoverable in solutions amenable to being further concentrated, without the generation of hazardous sludges or other harmful waste products.
For example, conventional toxic metal recovery technologies have been inefficient when applied to solutions having metal concentrations ranging from less than 1 mg/L to about 100 mg/L.
In the chemical area of toxic metal recovery from dilute aqueous streams, the techniques of recovery have most commonly been by chemical precipitation, ion exchange, reverse osmosis, electrodialysis, solvent extraction (liquid ion exchange), and chemical reduction (Rich, G., and K. Cherry. Hazardous Waste Treatment Technologies. Pudvan Publishing Co., 1987, 169 pp.); however, these procedures are characterized by the disadvantages of incomplete metal removal, high reagent and energy requirements, and generation of toxic sludge or other waste products that must be disposed of, and these disadvantages are particularly conspicuous at the low metal concentrations often encountered in waste waters, where federally-mandated cleanup standards dictate that effluents discharged to public waters contain &lt;1 mg/L of metals such as arsenic, cadmium, lead, mercury, iron and manganese.
On the other hand, although living microbial populations have shown some promise in extracting metals from these solutions (Jennett, J. C., J. E. Smith, and J. M. Hassett. Factors Influencing Metal Accumulation by Algae, NTIS PB83-149377, 124 pp.), (Ngo, V., and W. Poole, Boosting Treatment Pond Performance. Pollution Eng., v. 19, No. 9, Sept 1987, pp. 62-63), Burton, M. A. S., and P. J. Peterson. Metal Accumulation by Aquatic Bryophytes From Polluted Mine Streams. Environ. Pollution, v. 19, No. 1, May 1979, pp. 39-46) and (Tsezos, M., The Selective Extraction of Metals From Solution by Microorganisms: A Brief Overview. Can. Metall. Q., v. 24, No. 2, June 1985, pp. 141-144), it has been found that the maintenance of a healthy microbial population is often difficult owing to the toxicity of the aqueous streams processed, and recovery of the metal-laden microorganisms from solution is also troublesome due to liquid-solid separation problems.
Colonization of polymeric and foam supports with living microorganisms have also received attention, but problems such as nutrient requirements, toxic shock, and recovery of metal values from these supports still exist (Frenay, J. et al. Microbial Recovery of Metals From Low-Grade Materials. Paper and Recycle and Secondary Recovery of Metals, ed. by P. R. Taylor, H. V. Sohn, and N. Jarrett. Pub. by The Metall. Soc., Inc., 1985, pp. 275-278).
It has been found that, immobilization of thermally-killed biomass in a granular or gel matrix (Brierley, J. A. et al. Treatment of Microorganisms With Alkaline Solution to Enhance Metal Uptake Properties. U.S. Pat. No. 4,690,894) and (Nakajima, A., T. Horidoshi, and T. Sakaguchi. Studies on the Accumulation of Heavy Metal Elements in Biological Systems. XXI. Recovery of Uranium by Immobilized Microorganisms. Eur. J. Appl. Microbiol. and Biotechnol., v. 16(2-3), 1982, pp. 88-91) eliminates the need to supply nutrients and the problem of toxic shock, however, these materials are subject to fracture and attrition after repeated loading-elution cycles, especially in acidic solutions. As the granules and gels physically deteriorate, maintenance of aqueous flows in downflow columns is difficult since the fine particles impede the flow. Additionally, recovery of metal values from the metal-laden granules is often difficult.
Solvent extraction has also shown promise for extracting metal values from dilute aqueous solutions, but solubility and entrainment losses of the organic reagents in the aqueous phase result in high reagent costs and contaminate the metal-free aqueous stream with organic compounds.
Attempts have also been made to immobilize solvent extraction reagents in polymeric substrates such as membranes (Nichols, L. D., A. S. Obermayer, and M. B. Allen. Bound Liquid Ion Exchange Membranes For Recovery of Chromium From Wastewater. NTIS PB82-250580, 1980, 33 pp.), and although losses of organic reagent were lower than those encountered with conventional liquid-liquid solvent extraction, the organic reagents slowly desorbed from the beads into the aqueous processing solutions. Also, organic-aqueous complexes frequently precipitated within the substrate pores and plugged the membranes.
Accordingly, there is a need for new and innovative technologies in the mineral processing industry to recover toxic metal values from the low-grade aqueous solutions encountered.